In recent years, flat panel display devices have been developed and widely used in electronic applications such as personal computers. One of the popularly used flat panel display device is an active matrix liquid crystal display which provides improved resolution. However, the liquid crystal display device has many inherent limitations that render it unsuitable for a number of applications. For instance, liquid crystal displays have numerous fabrication limitations including a slow deposition process for coating a glass panel with amorphous silicon, high manufacturing complexity and low yield for the fabrication process. Moreover, the liquid crystal display devices require a fluorescent backlight which draws high power while most of the light generated is wasted. A liquid crystal display image is also difficult to see under bright light conditions or at wide viewing angles which further limit its use in many applications.
Other flat panel display devices have been developed in recent years to replace the liquid crystal display panels. One of such devices is a field emission display (FED) device that overcomes some of the limitations of LCD and provides significant advantages over the traditional LCD devices. For instance, the field emission display devices have higher contrast ratio, larger viewing angle, higher maximum brightness, lower power consumption and a wider operating temperature range when compared to a conventional thin film transistor (TFT) liquid crystal display panel.
One of the most drastic difference between a FED and a LCD is that, unlike the LCD, FED produces its own light source utilizing colored phosphors. The FEDs do not require complicated, power-consuming backlights and filters and as a result, almost all the light generated by a FED is visible to the user. Furthermore, the FEDs do not require large arrays of thin film transistors, and thus, a major source of high cost and yield problems for active matrix LCDs is eliminated.
In a FED, electrons are emitted from a cathode and impinge on phosphors on the back of a transparent cover plate to produce an image. Such a cathodoluminescent process is known as one of the most efficient methods for generating light. Contrary to a conventional CRT device, each pixel or emission unit in a FED has its own electron source, i.e., typically an array of emitting microtips. A voltage difference existed between a cathode and a gate extracts electrons from the cathode and accelerates them toward the phosphor coating. The emission current, and thus the display brightness, is strongly dependent on the work function of the emitting material. To achieve the necessary efficiency of a FED, the cleanliness and uniformity of the emitter source material are therefore very important.
In order for the electron to travel in a FED, most FEDs are evacuated to a low pressure, such as 10.sup.-7 torr, in order to provide a log mean free path for the emitted electrons and for preventing contamination and deterioration of the microtips. The resolution of the display can be improved by using a focus grid to collimate the electrons drawn from the microtips.
In the early development for field emission cathodes, a metal microtip emitter of molybdenum was utilized. In such a device, a silicon wafer is first oxidized to produce a thick silicon oxide layer and then a metallic gate layer is deposited on top of the oxide. The metallic gate layer is then patterned to form gate openings, while subsequent etching of the silicon oxide underneath the openings undercuts the gate and creates a well. A sacrificial material layer such as aluminum is deposited to prevent deposition of molybdenum into the emitter well. Molybdenum is then deposited at normal incidence such that a cone with a sharp point grows inside the cavity until the opening closes thereabove. An emitter cone is left when the sacrificial layer of aluminum is removed.
In an alternate design, silicon microtip emitters are produced by first conducting a thermal oxidation on silicon and then followed by patterning the oxide and selectively etching to form silicon chips. Further oxidation or etching protects the silicon and sharpens the point to provide a sacrificial layer. In another alternate design, the microtips are built onto a substrate of a desirable material such as glass, as an ideal substrate for large area flat panel display. The microtips can be formed of conducting materials such as metals or doped semi-conducting materials. In this alternate design for a FED device, an interlayer that has controlled conductivity deposited between the cathode and the microchips is highly desirable. A proper resistivity of the interlayer enables the device to operate in a stable condition. In fabricating such FED devices, it is therefore desirable to deposit an amorphous silicon film which has electrical conductivity in an intermediate range between that of intrinsic amorphous silicon and n.sup.+ doped amorphous silicon. The conductivity of the n.sup.+ doped amorphous silicon can be controlled by adjusting the amount of phosphorous atoms contained in the film.
Generally, in the fabrication of a FED device, the device is contained in a cavity of very low pressure such that the emission of electrons is not impeded. For instance, a low pressure of 10.sup.-7 torr is normally required. In order to prevent the collapse of two relatively large glass panels which form the FED device, spacers must be used to support and provide proper spacing between the two panels. For instance, in conventional FED devices, glass spheres have been used for maintaining such spacings in FED devices. For high anode voltage FED devices, elongated spacers have also been used for such purpose as shown in FIGS. 1A and 1B.
FIG. 1A is a perspective, partially exploded view of a conventional FED device 10. The FED device 10 is constructed by an upper glass plate 12 and a lower glass plate 14. In-between the two glass plates 12, 14, a plurality of elongated spacers 20 are utilized to support the spacing between the two plates under high vacuum pressure. The plurality of spacers 20 are held in place, i.e., positioned between active regions 16 formed on the surface 22 of the bottom glass plate 14. The plurality of elongated spacers 20 are held in place by slots 24 provided in sidewall panels 18, as shown in FIG. 1A and in an enlarged top view of FIG. 1B.
The conventional method for mounting the plurality of spacers 20, shown in FIGS. 1A and 1B, presents a number of processing difficulties. First, since the elongated spacers are not held in place at its center, the center portion of the spacer may easily be displaced from its correct position on the bottom glass plate. Furthermore, the elongated spacers 20 must be provided with vacuum passageways such that vacuum may be withdrawn in the cavity. Thirdly, slots at precise locations must be provided in the sidewall panels 18 which further complicates the fabrication process for the FED device.
In modem FED devices, higher operating voltages are frequently needed in order to achieve improved resolution and brightness of the device. For instance, a high voltage of several thousand volts is frequently employed as the driving voltage for the FED. At such high voltages, the spacing (0.5.about.5 mm) between the upper glass plate and the lower glass plate must be sufficiently maintained in order to avoid electrical discharges from occurring between the plates. The proper spacing in a FED cavity is therefore a more critical issue in high voltage FED devices.
It is therefore an object of the present invention to provide a method for forming a vacuum display device that does not have the drawbacks or shortcomings of the conventional methods.
It is another object of the present invention to provide a method for forming a vacuum display device such that the device can stand up to a high vacuum pressure without collapsing.
It is a further object of the present invention to provide a method for forming a vacuum display device designed for high operating voltage without incurring electrical discharge problems in the cavity.
It is another further object of the present invention to provide a method for forming a vacuum display device that has a spacing between two glass plates as large as 5 mm.
It is still another object of the present invention to provide a method for forming a vacuum display device by utilizing elongated spacers for maintaining the spacing between two parallely positioned glass plates.
It is yet another object of the present invention to provide a method for forming a vacuum display device by utilizing elongated spacers that are glued at a bottom edge to the lower glass plate.
It is still another further object of the present invention to provide a method for forming a field emission display device by utilizing elongated spacers which are mounted to a lower glass plate by utilizing a holding fixture to coat the bottom edges of the spacers with an adhesive.
It is yet another further object of the present invention to provide a method for fabricating a field emission display device utilizing elongated spacers by first clamping the spacers in a holding fixture and then coating the bottom edges of the spacers with a layer of adhesive previously deposited on a substrate by a screen printing or other coating methods.